1. Field of the Invention
The present invention relates to processes for forming aluminide coatings. More particularly, this invention relates to a process of forming an aluminide coating on a surface of a component having an existing thermal barrier coating without causing spallation of the thermal barrier coating.
2. Description of the Related Art
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc. Aluminum-containing coatings, particularly diffusion aluminide coatings, have found widespread use as environmental coatings on gas turbine engine components. Aluminide coatings are generally formed by a diffusion process such as pack cementation or vapor phase aluminizing (VPA) techniques, or by diffusing aluminum deposited by chemical vapor deposition (CVD) or slurry coating. During high temperature exposure in air, an aluminide coating forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the coating and the underlying substrate.
Slurry coatings used to form aluminide coatings contain an aluminum powder in an inorganic binder, and are directly applied to the surface to be aluminized. Aluminizing occurs as a result of heating the component in a non-oxidizing atmosphere or vacuum to a temperature that is maintained for a duration sufficient to melt the aluminum powder and diffuse the molten aluminum into the surface. As described in U.S. Pat. No. 6,444,054, slurry coatings may contain a carrier (activator), such as an alkali metal halide, which vaporizes and reacts with the aluminum powder to form a volatile aluminum halide, which then reacts at the component surface to form the aluminide coating. Because the thickness of an aluminide coating produced by a slurry method is proportional to the amount of the slurry applied to the surface, the amount of slurry applied must be very carefully controlled. While the presence of a halide is believed to displace oxides on the surface being treated, making it more likely that a clean uniform diffusion coating will result, the inability of slurry processes to consistently produce diffusion aluminide coatings of uniform thickness has discouraged their use on components that require a very uniform diffusion coating and/or have complicated geometries, such as turbine blades.
In contrast to slurry processes, pack cementation and VPA processes are widely used to form aluminide coatings because of their ability to form coatings of uniform thickness. Both of these processes generally entail reacting the surface of a component with an aluminum halide gas formed by reacting an activator (e.g., an ammonium or alkali metal halide) with an aluminum-containing source (donor) material. In pack cementation processes, the aluminum halide gas is produced by heating a powder mixture comprising the source material, the activator, and and inert filler such as calcined alumina. The ingredients of the powder mixture are mixed and then packed and pressed around the component to be treated, after which the component and powder mixture are heated to a temperature sufficient to vaporize the activator, which reacts with the source material to form the volatile aluminum halide, which then reacts at the component surface to form the aluminide coating. In contrast to pack processes, VPA processes are carried out with the source material placed out of contact with the surface to be aluminized. The source material can be an aluminum alloy or an aluminum halide, the latter of which eliminates the requirement for a separate activator. Aside from certain limited exceptions, such as a pack cementation process taught by U.S. Pat. No. 5,254,413 to Maricocchi and a VPA process taught by U.S. Pat. No. 6,326,057 to Das et al., both of which are assigned to the assignee of this invention, all conventional pack cementation and VPA processes have required the use of halide carriers or activators.
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, require some form of thermal protection in addition to an environmental coating. One approach is to deposit a thermal barrier coating (TBC) on the external surfaces of the component. TBC's must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. Coating systems capable of satisfying these requirements generally comprise a ceramic layer adhered to the component surface with an aluminum-containing bond coat, such as a diffusion aluminide coating or, more typically, an overlay coating deposited by thermal spraying or a physical vapor deposition (PVD) technique. At times, TBC is intentionally applied on limited surface regions of a component, such as those surface exposed to particularly high temperatures. TBC may also be unintentionally applied to limited regions of the component surface if TBC deposition is blocked because of the part geometry, as can happen with multi-airfoil vanes. In these cases, an aluminide coating can be applied to all exposed surfaces of a component prior to TBC deposition in order to protect those surfaces not protected by the TBC.
Another approach for providing thermal protection is to configure the component to provide cooling air flow through internal passages within the component, such that heat is absorbed from the external surfaces through the component walls. Certain air-cooled components are designed so that the cooling air is released into the gas path at specific locations on the component surface to provide a layer of cooling air flow over the component surface, creating a boundary layer (film) that reduces heat transfer from the hot gas path to the component. Temperatures inside internal cooling passages can be sufficiently high to require a diffusion aluminide coating for oxidation protection.
For more demanding applications, it becomes necessary to utilize internal cooling in combination with a TBC on the external surfaces of a gas turbine engine component. Particular examples are those components that are subjected to temperatures that exceed the melting temperature of the alloy from which they are formed. However, the size and geometry of film cooling holes for an air-cooled component are critical to maintaining the required amount of coolant flow, as well as the air flow pattern required to create the desired film boundary. If cooling holes are formed before any external coatings are applied, the final configuration of the hole opening is difficult to maintain and measure. For example, thermally-sprayed bond coats are typically deposited in one or more applications having tolerances that may be on the order of about 20% to 30%. TBC's applied by some form of thermal spray process, such as plasma spraying, high velocity oxy-fuel (HVOF), etc., are also inherently difficult to control on a local scale. As a result, the precise size and shape of a film cooling hole and other small, well-defined, features present in a component surface are lost, blurred, or otherwise altered by the subsequent deposition of a protective coating. On the other hand, processing complications are encountered if cooling holes are formed after the deposition of a protective coating. For example, bond coats are formed of hard, brittle materials that are very difficult to machine. Furthermore, the aluminide coatings desired for the internal cooling passages cannot be deposited after TBC deposition because the halide activator required by aluminide coating processes traditionally suitable for gas turbine components are incompatible with TBC materials. A TBC exposed during such an aluminizing process de-bonds or spalls from the component, leaving the area underneath with little or no thermal protection. Consequently, gas turbine engine components requiring both air cooling and TBC for thermal protection have been designed so that their cooling holes are properly sized after bond coat and TBC deposition, or their holes must be reopened after TBC deposition with the risk of damage to the aluminide coating protecting the cooling hole. Such damage to the internal aluminide coating is virtually impossible to detect and can lead to premature failure of the component.
In view of the above, it can be appreciated that the ability to combine cooling air flow, TBC, and aluminized cooling holes in the same component has not been fully realized because aluminized cooling holes are prone to damage when attempting to reestablish their shape and size after bond coat and TBC deposition, and the aluminizing of cooling holes after bond coat and TBC deposition is prohibited by the reaction that occurs between the halide and TBC. As a result of the latter, virtually all TBC-coated air-cooled gas turbine engine components equipped with film cooling holes have been manufactured according to the following sequence: machine the cooling holes; aluminide coat the cooling holes; and then deposit the TBC over the pre-machined, pre-coated, cooling holes, with the result that the cooling holes are at least partially blocked with TBC. It is believed that all prior attempts to rearrange the manufacturing sequence so that cooling hole machining and internal aluminide coating are performed after TBC deposition have failed because of the incompatibility of the halide activator and TBC materials.
Because TBC's are frangible, TBC-coated components are at risk of damage from handling that can lead to the loss of thermal protection, resulting in a local increase in component temperature during engine operation that may be unacceptable if the chipped region is sufficiently large. In this case, the TBC must typically be stripped from the entire component and reapplied. If the component is air-cooled, the reapplied TBC must be removed from the cooling holes in order to reestablish their size and geometry. As previously discussed, any aluminide coating present within a cooling hole that must undergo refurbishment in this manner is prone to damage and even removal when attempting to remove the TBC blocking the hole. Therefore, in addition to the desirability of combining different forms of thermal protection, it would be advantageous if, during the repair of a TBC-coated component, the oxidation resistance of the exposed bond coat could be enhanced by local application of an aluminide, instead of completely stripping and recoating the entire component. Furthermore, it would be advantageous if a component having TBC applied to only limited external surfaces (whether intentional or not) could be aluminized after TBC deposition to provide environmental protection on those surfaces not covered by the TBC. However, each of these capabilities has also been frustrated by the incompatibility between TBC and the halide activators used in aluminizing processes.